822 PATHOGENICITY 



E. histolytica in culture, and not the amoeba, which lost virulence during 

 the year in the artificial medium and regained virulence after repeated 

 liver passage. They add the precautionary remark that failure to infect 

 livers with cultures containing only nonpathogenic bacteria might not 

 be repeated with more virulent strains of E. histolytica. 



Frye and Meleney (1933) attacked the same problem, using severity 

 of intestinal infection as the criterion. They, too, tried a crisscross tech- 

 nique, interchanging the bacteria in a culture of proved high virulence 

 with those in a culture of proved low virulence. The interchange did not 

 materially alter the incidence or severity of infection of the two strains 

 of amoebae; hence their conclusion that the difference in pathogenicity 

 of the two cultures was really due to the amoebae themselves. Thus 

 differences in pathogenicity of strains of E. histolytica claimed by Mel- 

 eney and Frye was shown to be due to inherent qualities of the proto- 

 zoon, and not to accompanying microorganisms. Since they had previously 

 not been able to detect any alteration of pathogenicity in artificial me- 

 dium, the question of alteration of virulence of bacteria in such a medium 

 does not enter in. 



MALARIA: Plasmodium vivax 



It is inescapable that there are strains of intestinal Protozoa differing 

 in virulence, but what is the situation regarding the pathogenic blood 

 Protozoa.'' Since the behavior of the trypanosomes in animal passage is 

 complicated by differences in behavior of "passage" and "relapse" 

 strains, the author prefers to evade discussing this subject. Human 

 malaria, however, lends itself more readily to discussion, as becomes 

 evident after reading the chapter entitled "The Complexity of the Ma- 

 laria Parasite" in Hackett (1937). Malaria therapy in general paralysis 

 (paresis) has made it possible to determine definitely whether there are 

 strains of the human malarias differing in morphology, pathogenicity, or 

 other behavior, and the facts learned have been rather surprising. 



Plasmodium vivax is the species commonly employed in malaria ther- 

 apy. Using infected Anopheles for inoculation, Boyd and Stratman- 

 Thomas (1933a) showed that during an attack of malaria induced by a 

 particular strain of this species, a patient acquires a "tolerance" which 

 makes him refractory to reinoculation with that strain, but not with a 

 different strain of the same species. They concluded that a person in- 



